I am going to skip all the detail and simply say that ISA 2004 completely died and stopped routing traffic due to crappy Broadcom drivers. Even top level Microsoft support had troubles tracking it down to bad drivers.
On Susan’s blog, users have said that uninstalling the Dell preinstalled drivers and installing the Broadcom drivers fixed the issue. At the time however, this did not work for me… unless they released updated drivers since Dec 2006??
Anyway, my recommendation is to simply request Intel network cards when you order your Dell server. Just to be on the safe side.